The present invention relates to a method and apparatus for cleaning sediment and fine debris that has been accumulated around ingress and egress piping within water-submerged portions of existing nuclear reactor pressure vessels. More particularly, the invention relates to a manually manipulated cleaning tool which may be temporarily clamped around submerged piping at the reactor vessel walls and operated to seal an area in which debris has collected for purposes of cleaning. Remote visual monitoring and control of the cleaning apparatus is contemplated. The method of the present invention involves the manipulation of the cleaning apparatus to a desired location, the sealing of the contaminated area and the cleaning of the area using hydraulic, pneumatic and/or mechanical operations.
The steam and power conversion system of a nuclear power plant is similar in basic design to that of a conventional steam plant and consists of a basic steam loop, auxiliary systems, and the turbine generator. In a typical configuration, water is circulated through a boiling water nuclear reactor where it is converted to steam. Steam is passed to a turbine generator where it produces electricity and is then condensed and circulated back to the boiling water reactor. The boiling water reactor comprises a reactor vessel that contains a reactor core as a heating source as well as steam separator apparatus and steam dryer apparatus that prepares the steam for delivery to the turbines. The internal apparatus of the reactor pressure vessel generally are removable for repair.
Nuclear reactor pressure vessels that are used commercially for the production of electrical power require periodic inspections as a result of legislation and regulations established by federal and state governments, as well as individual utility policies. During such inspections, the reactor is shut down and certain components that occupy the reactor pressure vessel volume are removed, including the steam dryer and moisture separator in the top portion and the fuel itself. Coolant remains in the pressure vessel as a protection against radiation, however the recirculation system generally is not operated during shut-down. A detail inspection of the physical integrity of the operating system and reactor components is conducted. In particular, inspections are made to detect damage to the operating components of the reactor vessel, including those which are at all times completely submerged in coolant during both operation and inspection. An effort also is made to identify cracks or other possible areas that may jeopardize the safe operation and/or force emergency shut down of the reactor. The inspection extends to the feedwater spargers, core spray mechanisms, water inlet nozzles and internal reactor pressure vessel components.
The standard design of nuclear reactor vessels and, in particular, their coolant circulation systems, include filtering systems which are intended to remove suspended contaminants. However, because of the path taken by the coolant within the vessel during normal operations, turbulent flow occurs as the coolant passes across or around the submerged piping and jet pump assemblies. As is well-known from fluid flow mechanics, suspended particles will tend to accumulate in areas that are not in the path of direct flow. One group of such areas that is of particular concern to the present invention involves the piping that penetrates the reactor pressure vessel, such as recirculation inlet nozzles that provide coolant to the jet pumps and recirculating coolant outlets. Typically, the penetration interfaces for piping or other equipment and the reactor pressure vessel are designed with horizontal spacing that are out of the direct flow of circulating water and provide a depository for sediment and debris.
The task of cleaning debris and sediment from these penetration areas has proven to present great difficulties. While conventional underwater vacuuming systems may be used to clean around exposed structures within the vessel, these interface areas have presented an insurmountable problem.
Concern for contamination of the pressure vessel environment and, in particular, the vessel coolant, has limited candidate cleaning apparatus to station supplied utilities, including shop air (85-100 PSIG) and shop power (120V AC).
Economic considerations also demand that the cost of any cleaning system be spread as broadly as possible. Since any given facility may have several reactor pressure vessels, the use of permanent cleaning systems for each vessel is not attractive because of the added cost.
Accordingly, it is a primary object of the present invention to provide a reactor pressure vessel cleaning and decontamination system which may be easily manipulated from a remote location to clean the interface areas between external piping and the pressure vessel walls while coolant remains within the vessel.
Another object is to provide a remotely operable and easily moveable set of tools that are designed to clean interface areas of different sizes and shapes.
Yet another object is to provide a compact, easily assembled and dissembled, and portable penetration interface cleaning apparatus that uses local plant power and air for operation. The tools may be sized and shaped, within the basic concept of the present invention, to adopt to a variety of penetration interface areas and in vessels at the same or different plant sites.
A further object is to provide a cleaning tool, which may be clamped about a pipe interfacing with a wall surface, to seal an area to be cleaned and to conduct a cleaning operation.
Still another object is to provide a method for maneuvering a tool to a particular pipe and wall interface within a nuclear reactor vessel, to seal off an area to be cleaned, and to clean the area using a variety of hydraulic, pneumatic and mechanical operations.